hugin has a brightness and colour correction system which is completely
independent of the old PTStitcher tool. This improved system is only
available with the default nona stitching tool in the hugin Stitcher tab.

The hugin system works by sampling a spread of points for each pair of
overlapping images. The optimiser then tries to model a system of camera response curve,
exposure, white balance and vignetting that fits the values of these points.

So for this to work, the photos in the project have to be already aligned. Align
photos by managing control points in the hugin Control Points tab and optimising
geometric image parameters in the hugin Optimizer tab.

Optimize Preset

Use the Optimize Preset combo box to pick one of several pre-set photometric
optimisation schemes, then click the Optimize now! button to calculate the
best available brightness and colour adjustments.

High dynamic range, variable white balance, fixed exposure

Custom parameters below

The pre-set options are good for most situations, but often it is necessary to switch to
Custom parameters. For example, none of the pre-sets will optimise the
Vignetting Centre, so use custom parameters if your vignetting is off-centre.

Image Variables

Image variables are quite likely to vary between photos, perhaps because of minor
variations in shutter speed, changes in natural light or because of 'auto' settings
in the camera itself.

Exposure

The Exposure section shows the photo number and exposure values for all input
photos (in parenthesis), the check mark indicates parameters that will be optimised.

When a value is set to 0 (zero) this results in hugin applying no exposure
change to the photo. EV is a standard photographic scale, each increase or
decrease by one unit will change the exposure by the equivalent of one f-stop
(ie. halving or doubling the exposure).

White balance

Also known as colour balance or colour temperature.

The White balance section shows the photo number and red and blue
multiplier values (in parenthesis), the check mark indicates photos that
will be optimised. If the values are set to (1,1), this will result in no
white balance change (the numbers are relative to the green channel which
stays unaltered).

Vignettting

Vignetting is dependent mainly on your lens and the aperture. Usually the centre of the image is
brighter with a falloff towards the edges, hugin can calculate this falloff curve as part of the
photometric optimisation process or you can enter it manually in the hugin Camera and Lens tab
as the three numbers shown here.

Vignetting Centre

The centre of vignetting is rarely the exact centre of the photo,
hugin can optimise this position or you can enter it manually in the
hugin Camera and Lens tab. The scale is in pixels, with 0,0
indicating the centre of the photo. The values are independent of the
d & e parameters, that specify the origin for projection and distortion
values.

Camera Response

hugin can optimise the camera response curve by comparing differences
between overlapping images. To do this your photos need to either have
significant vignetting or have variable exposure. If your photos have
perfectly even exposure and zero vignetting, then you would have to
calibrate the camera response separately and then enter it manually in the
hugin Camera and Lens tab.

The camera response curve is used both for mapping the images to a linear
colourspace when creating HDR output, and for normalising the colourspace
for internal vignetting, brightness and colour corrections when creating
'normal' LDR output. If your pictures don't require such corrections
then you don't really need a calibrated response curve.

Hugin uses the EMoR response model
from the Computer Vision Lab at Columbia University which simplifies the full
response curve to five empirical coefficient numbers.

If your brightness variations are caused by lens flare then you may be better not optimising
Exposure or white balance, instead uncheck Link for Camera Response
in the hugin Camera and Lens tab to optimise a different response curve for each photo.